One-process models of recognition memory assume that judgments are based on the familiarity of the test probe. Two-process models assume that the familiarity-based process may be supplemented by the output of a recollective process. One type of evidence for two-process models has come from subjective reports of whether a particular test item is recognized on the basis of recollection or familiarity, using a remember-know (R-K) procedure (Tulving, 1985). Quantitative modeling has been applied to R-K judgments only recently, and a number of candidate models have emerged. Like models of recognition, models of R-K come in one- and two-process forms. The one-dimensional signal detection (SDT) explanation of R-K judgments was first proposed by Donaldson (1996) but has been extended by Wixted and Stretch (2004). We proposed a two-dimensional SDT model known as STREAK (Rotello, Macmillan, &Reeder, 2004). Other two-process models have been suggested by Reder et al. (2000), Yonelinas (2002), and Murdock (in press). We focus on distinguishing these alternative models, using ROC curves and analysis of RT distributions as the primary methodological tools. Aims 1 and 3 evaluate the source of the variability assumed in SDT models. In recognition tasks, variability arises from differences among the stimuli, and models typically treat all kinds of variability alike. That this is an obvious oversimplification in the R-K paradigm: surely some items are more likely to lead to R responses than others. We address this issue by varying stimulus characteristics in our typical word-recognition experiments (Aim 1), and by studying memory for faces (Aim 3). Aim 2 takes a different approach to model evaluation, focusing on the RT distributions for "old" responses that are followed by R or K judgments. The statistics of these distributions are informative about whether the same process(es) underlie the judgments. We take advantage of this fact to test models of R-K decisions. This is clearly a basic research program. However, understanding the mechanisms by which recognition judgments are made in healthy adults is critical to understanding the problems that can develop after brain injury or in later life. This research will have implications for amnesia, aging effects such as cognitive slowing, and prosopagnosia (the inability to recognize faces);it is also relevant for eyewitness memory.